Laptop computers contain processors and memory, which in many cases are comparable in terms of performance with those in desktop machines. But the hard drives in laptop computers are typically significantly slower than those in desktop machines. This is because of the constraints in terms of power, cooling, and vibration tolerance that are required of a laptop computer.
As a result, hard drive performance is often the most significant limit in performance of laptop computers, when compared to desktop machines. This can be observed, when using a portable computer, in the time taken to boot a system, or in the time taken to execute a data intensive application. Often, these limits in performance are perceived for short periods only such as when an application is started, with the drive idle for significant periods in between. But, these periods are costly because the user is often left waiting for their duration.
It is possible to boot a laptop from a higher performing hard drive via a docking station, and then receive the performance of that class of drive. But, once booted, it is not possible to remove the laptop from the docking station without shutting down. Once undocked, the data in the hard drive is inaccessible.